Dissection of prostate showing the fibers of the external sphincter surrounding the membranous urethra and partially cradling the inferior portion of the prostate.
The external urethral sphincter originates at theischiopubic ramus and inserts into the intermeshing muscle fibers from the other side. It is controlled by the deep perineal branch of thepudendal nerve. Activity in the nerve fibers constricts the urethra.
Theinternal sphincter muscle of urethra: located at the bladder's inferior end and the urethra's proximal end at the junction of the urethra with the urinary bladder. The internal sphincter is a continuation of thedetrusor muscle and is made ofsmooth muscle, therefore it is underinvoluntary orautonomic control. This is the primary muscle for prohibitingurination.
Thefemale ormale external sphincter muscle of urethra (sphincter urethrae): located in thedeep perineal pouch, at the bladder's distal inferior end in females, and inferior to the prostate (at the level of themembranous urethra) in males. It is a secondary sphincter to control the flow of urine through the urethra. Unlike the internal sphincter muscle, the external sphincter is made ofskeletal muscle, therefore it is under voluntary control of the somatic nervous system.[1]
In males and females, bothinternal and external urethral sphincters function to prevent the release of urine. The internal urethral sphincter controls involuntary urine flow from the bladder to the urethra, whereas the external urethral sphincter controls voluntary urine flow from the bladder to the urethra.[2] Any damage to these muscles can lead tourinary incontinence. In males, the internal urethral sphincter has the additional function of preventing the flow ofsemen into the male bladder duringejaculation.[3]
Females do have a more elaborate external sphincter muscle than males as it is made up of three parts: the sphincter urethrae, the urethrovaginal muscle, and the compressor urethrae. The urethrovaginal muscle fibers wrap around the vagina and urethra and contraction leads to constriction of both the vagina and the urethra. The origin of the compressor urethrae muscle is the right and leftinferior pubic ramus and it wraps anteriorly around the urethra so when it contracts, it squeezes the urethra against the vagina. The external urethrae, like in males, wraps solely around the urethra.[4]
Congenital abnormalities of the female urethra can be surgically repaired withvaginoplasty.[5]
The urethral sphincter is considered an integral part of maintainingurinary continence, and it is important to understand its role in some conditions:
Stress urinary incontinence is a common problem related to the function of the urethral sphincter. Weak pelvic floor muscles, intrinsic sphincter damage, or damage to the surrounding nerves and tissue can make the urethral sphincter incompetent, and subsequently it will not close fully, leading to stress urinary incontinence. In women, childbirth, obesity, and age can all be risk factors, especially by weakening the pelvic floor muscles.[6] In men, prostate surgery (prostatectomy,TURP, etc) and radiation therapy can damage the sphincter and cause stress incontinence.[7]
^Maclean, Allan; Reid, Wendy (2011). "40". In Shaw, Robert (ed.).Gynaecology. Edinburgh New York: Churchill Livingstone/Elsevier. pp. 599–612.ISBN978-0-7020-3120-5; Access provided by the University of Pittsburgh{{cite book}}: CS1 maint: postscript (link)
^abGupta S, Sharma R, Agarwal A, Parekh N, Finelli R, Shah R, Kandil H, Saleh R, Arafa M, Ko E, Simopoulou M, Zini A, Rajmil O, Kavoussi P, Singh K, Ambar RF, Elbardisi H, Sengupta P, Martinez M, Boitrelle F, Alves MG, Khalafalla K, Roychoudhury S, Busetto GM, Gosalvez J, Tadros N, Palani A, Rodriguez MG, Anagnostopoulou C, Micic S, Rocco L, Mostafa T, Alvarez JG, Jindal S, Sallam H, Rosas IM, Lewis S, AlSaid S, Altan M, Park HJ, Ramsay J, Parekattil S, Sabbaghian M, Tremellen K, Vogiatzi P, Gilani M, Evenson DP, Colpi GM (April 2022)."A Comprehensive Guide to Sperm Recovery in Infertile Men with Retrograde Ejaculation".The World Journal of Men's Health.40 (2):208–216.doi:10.5534/wjmh.210069.PMC8987146.PMID34169680.
^Netter, Frank H. (2019).Atlas of Human Anatomy, Seventh Edition. Philadelphia: Elsevier.ISBN9780323393225.
Anatomy figure: 41:06-04 at Human Anatomy Online, SUNY Downstate Medical Center - "Muscles of the female urogenital diaphragm (deep perineal pouch) and structures located inferior to it."
This article incorporates text in thepublic domain from the 20th edition ofGray's Anatomy(1918)